H.E.S.S.

High Energy Stereoscopic System

HESS J1503-582 - a "Forbidden Velocity Wing"?

November 2008

The
HI 21 cm line
traces the distribution of
neutral hydrogen in the Galaxy; from the intensity and Doppler shift of the
line the amount of hydrogen gas and its velocity along the line of sight can be
determined. Due to the differential rotation of the Galaxy, the mean velocity
and velocity range vary with Galactic longitude, showing that regions of the
Galaxy seen around longitude 80-160 degr. move towards us, whereas the regions
around 20-60 degr. tend to move away, with a wide dispersion. The distribution
of velocities shows, however, also compact regions protruding from the band,
indicative of gas masses with large velocity dispersion, termed Forbidden Velocity
Winds (FVW)
(from Kang & Koo 2007).

Forbidden velocity wings (FVW, see top image) obviously
represent objects involving gas masses with high velocity dispersion, such as
supernova remnants or strong stellar winds. However, only a small fraction of
all FVWs as identified by
Kang & Koo (2007)
coincide with known supernova
remnants (SNRs) (Fig. 1). Previously unknown, old SNRs in
the radiative phase could be the most likely candidates, as in the case of the
discovery of the SNR associated with FVW 190.2+1.1
(Koo et al. 2006).

HESS J1503-582 (Fig. 2) is a 6-sigma
source candidate revealed in HESS data taken during the Galactic Plane Survey
and in observations of the nearby object
MSH 15-52.
The region was first
targeted in 2004 and was later observed regularly until 2007. HESS J1503-582
shows up as an extended gamma-ray excess at longitude 319.7 degr. and latitude 0.3 degr.,
with an rms size of about 0.3 degr. Two potential counterparts were found in
catalogs: the X-ray source AX J1504.6-5824, which is, however, catalogued
as a Cataclysmic Variable, and the very old pulsar PSR J1502-5828,
with an age of 0.3 Myr. Pulsar wind nebulae indeed represent a significant
fraction of all Galactic H.E.S.S. sources. The spin-down energy loss of this
particular pulsar is, however, one to two orders of magnitude too small to
plausibly account for the gamma-ray source. Interestingly, the forbidden
velocity wing FVW
319.8+0.3, marked with the highest detection rank by
Kang & Koo (2007), is
spatially coincident with HESS J1503-582, as shown in Fig. 3.
This FVW appears in the HI line image integrated between gas velocities of -123
to -98 km/s,
two velocities which are not permitted by the canonical Galactic rotation curve
along this line of sight. The exact nature of the source, however, remains
unclear.
Old supernova remnants with ages of some 10 kyr are not expected to accelerate multi-TeV
particles any longer, mainly because of the very low shock speed. On the other
hand, the joint activity of stellar winds and supernova explosions from massive stars
in nearby and powerful OB associations could produce fast-moving neutral
hydrogen gas detectable at the sensitivity level of the current HI surveys, and
serve to accelerate particles and create gamma rays, as seen in the source
HESS
J1023575 co-located with the Westerlund 2 cluster.

Reference: "On
the nature of HESS J1503-582 revealed by the H.E.S.S. experiment: Coincidence
with a FVW?",
H.E.S.S. collaboration, M. Renaud et al., to appear in Proc. of the 4th
Heidelberg International Symposium on High Energy Gamma-Ray Astronomy.

Fig. 1:
Distribution of FVWs in (ℓ,b) plane. The color of each symbol indicates the
mean velocity of the HI emitting gas and corresponds to the velocity in the
color bar on the top in km/s. Each symbol indicates a related object, and its
occupied area indicates the integrated antenna temperature as shown in the
bottom. The areas filled with solid black show gaps between data, and the
dotted line shows the boundary of the search in latitude. (From
Kang & Koo
2007)

Fig. 2:
Gamma-ray image of HESS J1503-582. Its peak significance reaches 6 sigma.
The black cross and circle denote the uncertainty of the source centroid and
its intrinsic size, respectively. The Galactic Plane is shown as the dashed
line. The bright source in the lower left corner is
MSH 15-52.

Fig. 3:SGPS
(Parkes) image of the HI line emission centered on FVW
319.8+0.3, velocity-integrated between -123 and -98 km/s . H.E.S.S. significance
contours are shown as 4 and 5 sigma levels. Note that the image is in Galactic
coordinates, rotated compared to Fig. 2.